Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the inkjets are controlled to emit ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
In some of these imaging devices, multiple printheads form an image as the image substrate passes the printheads only once. For example, a web of paper moving past a plurality of printheads receives the ink ejected from the printheads and then moves to a web heater and fixer for further treatment of the image. The multiple printheads may be arranged in a serial manner to provide an appropriate density of pixels per unit of linear measurement for a particular color. For example, two printheads may be arranged vertically in the direction of the web movement with each printhead having 300 inkjets per inch in each row of inkjets in the printhead. By offsetting the second printhead from the first printhead by a half-width of a single inkjet, the pair of printheads forms rows of printed pixels at a density of 600 dots per inch (dpi). In this arrangement, each printhead prints half of the pixels in each row of an image.
One issue affecting image quality in imaging devices having serially arranged printheads is misalignment of the printheads. Misalignment occurs when the two printheads are not precisely aligned either to provide the single inkjet offset between the printheads or in the direction of the web movement such that neighboring pixels from the two heads do not form a row of drops. In areas of an image that are comprised of pixels of the same color having approximately the same ink density, this misalignment can be detected by the human eye. In four color or CYMK systems, the imaging device may include a pair of serially aligned printheads for each color. Misalignments between printheads may be quite detectable by the human eye, especially in relatively uniform areas of the image. In previously known systems, various techniques have been developed to treat the image densities in a color separation to address such discrepancies, however, such systems have not necessarily been able to compensate for the lack of registration between serially arranged printheads for all image densities in a color separation.